This is a topic we have discussed before, the trend for shippers to use darker glass meaning fill levels cannot be determined.
This seems to me to have become more prevalent from the 1990's but I am sure there are members who have more knowledge here.
In a recent Sotheby's catalogue I noted a number of lots from the 1990's where written in the detail was ' Glass too dark to see levels' .
This in my view is extremely unfortunate and as these bottles get older and fill levels become more variable it is going to create serious issues for us as consumers.

1)I would be interested if anyone has any thoughts about how to try and work out the fill levels.
2) I am unsure why producers need to use such opaque glass ( I realise the reasons for darker glass ) and whether they could be encouraged to move back to glass that allows the consumer to know what they are buying, this must surely to be the benefit of producers as well.

In answer to question 1 I would say that as the glass has got darker the lights have got brighter. I find that a fierce LED cuts through most glass, although I have not tried this with Vesuviuo bottles. I have also for the most part given up shining light through the bottle while decanting. I use a silver funnel with a pierced strainer that allows me to see the sediment coming. I also find that decanting the bottle without standing it seems to let the crust hold together better. I too am a bit nonplussed by the fashion for effectively black glass. A lot of the older bottles I have are the colour of modern wine bottles and the port inside does not seem to have come to any harm. I don't know if there are any producers who still use green bottles.

Some bottles can certainly be tricky; while I have only ever come across 2 bottles for which I could not determine a level with my best torch for the purpose, I realise that many merchants and auctioneers are less motivated in finding the ideal torch and/or the time per bottle (for the torch it's not just about brightness, but also beam width, pattern and dispersion). I expect many manufacturers have simply used "as dark as possible without significant cost increase" on the grounds of maximum light protection.

There are other options which could be used reliably but most non-contact methods either expect to be in-vessel or be expensive such as laser/radar. Perhaps use of capacitance sensors might be possible fairly inexpensively.

I'd had the same thought.
I figured the options would likely be x-ray, ultra-wide-band, or perhaps a CT/MRI scanner (plus some clever algorithms to determine air volume to convert from side view to show level in a bottle when turned upright from imaging while on its side).

There does seem to be some interesting work out there on low-cost (lower resolution) x-ray and MRI. Some high-end auction houses might(?) already have access to such technology for other purposes, such as establishing authenticity for paintings/antiques?

(though a crowbar and decent torch is still a lot lower cost, and simpler to obtain)

Could a device be developed that could "see" inside an unopened OWC to report back on the levels of each of the dozen bottles regardless of the opacity of the glass?

I gave a lot of thought to this some years ago, and established that super powerful torches - even lasers - still fail to get through most opaque bottles. I have since settled on the practice of weighing and then re-weighing bottles a few years later.

This actually has an advantage over checking levels visually, as one is not then condemning bottles with sound closures whose only sin was to have a poor original fill.

However, if you wish to establish the level in an opaque bottle and don't have the odd spare million to invest in your own MRI, the most promising route I could establish - although I did not fully develop it.. - would be to have sprung calipers with a miniature ultrasonic transmitter and receiver attached.

If the receiver were then attached to an oscilloscope and the calipers were moved up and down the neck, the oscilloscope display should change when the signal was no longer deadened by the presence of fluid within the neck.

The biggest drawback though is that it would be difficult to get a meaningful reading between BN and VTS levels using this method.

Weighing bottles though is not that time consuming if you get yourself organised, and I don't find it any way a chore - in a strange way it enables you to 'know' your bottles better.

For weighing I use a laboratory balance designed for everyday school use, that weighs to an accuracy of 0.1g Although a longer interval is advantageous, it is possible to identify the weaklings in a case in as little as six months after the first weigh-in.

I may be drunk, Miss, but in the morning I shall be sober and you will still be ugly - W.S. Churchill

While weighing is certainly viable for long term monitoring of bottles (and cases?) in your possession, I think the original post was more targeted at assessment of unknown bottles for potential purchase.

I also considered ultrasound, but there would be a number of problems and risks:
- with the speed of ultrasound in air being 10x faster than through glass or water, preventing the otherwise dominant indirect path in air around the bottle would be tricky (fastest and least attenuated) which would likely swamp the heavily attenuated paths through glass, and through glass+air or glass+wine. Sufficiently preventing the air path would be very difficult given the surface contact variation around the bottle shoulder/neck.
- the variation in bottle width around the locations of potential interest also complicates such single measurement, whether using attenuation or time of flight measurement, though use of a pair of stimulus and sensor above and below for detection rather than moving single sensor might mitigate this (this would apply equally to many sensor types, not just ultrasound).
- risk of glass breakage, especially if any chips or cracks in old bottles
- potential effect on the wine (especially the lees); consider that ultrasound is used to break down and separate organic material.

I like the challenge to make the task easier (and may do some tests with a capacitive sensor at some point if I can find a suitably small one), but come back to the fact that there is likely minimal commercial justification in creation of such a device unless it can be done at very low cost, given that there are very few bottles for which the level cannot be determined with tenacity and the right torch (combination of side, beam and power, not just power!).

My idea for ultrasound use was an upmarket variant of tapping the bottle - transmitting and receiving the sound via metal probes in contact with the glass, so as to reduce the scope for air transmission spoiling the reading.

The capacitative idea though is very interesting, as the dielectric properties of a filled neck and empty neck will differ. It should be possible to make the capacitance control a simple audio oscillator circuit, to create a rising and falling tone as a pair of probes pass the level in the bottle..

I may be drunk, Miss, but in the morning I shall be sober and you will still be ugly - W.S. Churchill